This invention relates generally to a method of preparing a permanent master with a permanent latent image for use in electrostatic image transfer to a receiving substrate. More specifically, it pertains to a method of adjusting the conductivity of unexposed regions of a photopolymer so that fine image structure may be charged, developed and electrostatically transferred.
The use of an electrostatic printing plate or master employing a photopolymerizable composition has recently been developed for use in xeroprinting electrostatic transfer U.S. Pat. No. 4,732,831 to Riesenfeld et al discloses the use of special photopolymer compositions specifically designed or tailored to have excellent charge retention and charge decay characteristics that are also resistant to hydrocarbon solvents and are compatible with liquid toners. Riesenfeld designed specific formulations for use as a photopolymer master, but did not address the issue of utilizing existing photopolymer materials that were too conductive.
Another similar approach was taken in U.S. Pat. No. 3,286,025 to Ingersoll with a photopolymer material that used polymerized areas of reduced conductivity which could be corona charged on a conductive support and toned to form a toner image. This approach utilized the plate to store information, but required the washing out of non-imaged areas with a solvent. No transfer was involved in the Ingersoll approach. Ingersoll did involve a photopolymer that increased in resistivity or decreased in conductivity in the exposed areas.
Electrostatic master systems employing photopolymer materials which are exposed to actinic radiation to create the latent image have suffered from the problem of the variability in conductivity of the photopolymer material. This is especially a problem where fine line or fine image structure is required. The exposed areas will hold charge well and retain it for many seconds. However, unexposed regions of highly conductive photopolymers are so conductive that they cannot be charged to any significant value. This has been described as the "island effect" with regard to fine image structure. The island effect involves charge accumulation on fine lines or fine image structure, for example a 50 micron wide line in a background of relatively conductive unexposed photopolymer. The surface charge will initially accumulate in the fine image structure, but the electric fields parallel to the surface of the fine image structure will repel further charge and prevent sufficient charge from building up in the image structure to attract toner particles during the development stage.
This problem is solved in the method of the instant invention where intrinsincally conductive photopolymers are exposed in a step-wise fashion to make the background regions more resistive (less conductive) to permit fine image structure to be charged and subsequently developed and transferred.